Electric power systems have multiple generation units that operate in synchronism under a normal operation. That is, frequency, phase, and amplitude of voltages at the terminals of a generator hold a fixed relationship with the same parameters of the remaining generators in the power system. Before a generator can be connected to an electric power system, the frequency, phase, and amplitude of the voltages at its bus need to be matched with those of the power system at the point of interconnection. Once, the so called synchronization parameters are matched within a desired tolerance, the generator breaker is closed. Any mismatch in the synchronization parameters during connection of a generation unit by a generator breaker may result in undesired transients and disruption of the system.
Concept of microgrid, in which several small distributed generation units operate together to form a small power system, is finding increasing acceptance as a solution to increase the share of renewable energy resources. A microgrid may be operated in either of the following two modes: grid-connected mode and island mode. In grid-connected model, entire microgrid constituting several distributed generation units operate as a single generator from the perspective of the main grid. Hence, synchronization of a microgrid with the main grid is further challenging as the synchronization parameters of the microgrid at the point of interconnection with the main grid depends on several generation units. Synchronization process may also require communication among the distributed generation units in the microgrid.
There are several problems with the current synchronization method, both for single generation units, as well as for a microgrid with a group of distributed generation units. Synchronization is achieved using the frequency and voltage control of the generation units. Frequency control is responsible for fulfilling the frequency and phase matching requirements for the synchronization. Voltage control on the other hand is responsible for fulfilling the amplitude matching requirement. Speed of both the frequency and voltage control are slow in generation units as they are designed for a steady-state performance, synchronization is an auxiliary function for them and they are not designed for it. Moreover, for generation units based on rotating machines, the bandwidth of the frequency control is further limited by slow mechanical dynamics. The reliance on the controls of a generation unit for synchronization increases time required for connection the units with an electric power system.
Frequency control of generation units in power systems is usually implemented at different hierarchical levels to coordinate generation units distributed over a wide geography. These levels are generally referred as primary, secondary, and tertiary control. Primary control is implemented locally at the generation units and its bandwidth is limited by the generation unit characteristics. Secondary control is implemented at a central controller and it provides a frequency reference to the primary control of the generation units. Secondary control bandwidth is kept much lower than the primary control to avoid conflict between both these controls. Moreover, secondary control also relies on communication for coordination among the generation units. For microgrids, constituting several generation units, synchronization function is achieved by the secondary control for regulating the synchronization parameters at the point of interconnection of the microgrid with the main grid. It can be inferred from the above discussion that the speed of synchronization is severely constrained when it is achieved by the secondary control of the generation units, as in the current solution for the synchronization. In fact, synchronization using the frequency and voltage control of the generation units require several minutes. The slow synchronization may not be acceptable in future power systems where it may be desired to change the power system configuration dynamically and adaptively over time.
A problem with the current synchronization process for a group of distributed generation units in a microgrid is the requirement of communication among the generation units as the synchronization process is achieved using the secondary control.
Another problem with the current synchronization process is that the phase angle remains an uncontrolled variable. Moreover, it is kept uncontrolled even during the synchronization process, as discussed in U.S. Pat. No. 7,915,868 B1. Because of this, the phase matching requirement for the synchronization is achieved indirectly by maintaining a frequency difference between a generation unit or a microgrid and the electric power system. Due to the frequency difference, phase angle difference between the generation unit and the electric power system changes gradually and the generator breaker is closed when the phase angle difference is within a prescribed limit. This makes the synchronization time highly variable and dependent on the initial phase angle difference and the frequency offset. Higher frequency offset is required to increase the speed at which the phase angle difference evolves with time. However, this is detrimental to the synchronization performance as high frequency offset may result in large transient during interconnection. On the other hand, low frequency offset for phase synchronization takes exceedingly a long time due to the “waiting process” described in EP 2651000 A2.
Therefore, there is a need for developing fast synchronization of a generation unit or a microgrid with several generation units with an electric power system. Moreover, it is desired that the system should be located at the point of interconnection and should rely on measurement of only the local variables in order avoid communication requirement. Further, the system should actively regulate the phase angle during the synchronization process to eliminate the “waiting process”. Finally, the measurement method of the phase angle should be able to mitigate errors introduces by the harmonics and unbalance present in the three-phase voltages of the generation unit and the electric power system.